The present invention relates to a parallel-connected converter system, and more particular relates to a method of maintaining synchronized switching of the parallel converter system of multiple parallel-connected converters in various working conditions.
At present, the parallel-connected converters are commonly used. In the present parallel converter system, the following methods are used to control the switching of the multiple converters.
The first method is to use centralized control of the parallel-connected converters. As shown in FIG. 1, the parallel-connected converters are controlled by a common controller such that the switching of the converters is completely the same so as to overcome the high-frequency circulating current among the parallel-connected converters. As seen in FIG. 2, a common controller controls two parallel-connected converters such that the driving waveforms of the bridge legs of every converter are the same, and the voltage waveforms applied on the filtering inductors are the same. Therefore, no high-frequency circulating current exists between the filtering inductance of the converters. However, this method has a few defects such as nonflexible control, long control cables, and sensitive to interference, and thus this method is used only on the modular parallel converters, while it is not adopted in large parallel-connected power converters.
The second method is to control the parallel-connected converters separately. The switching frequency is constant, and it does not handle the high frequency circulating current. The switching patterns among such converters slides between two states of FIG. 3. In state 1 of FIG. 3, the high frequency voltage difference between the converters is zero when the switching waveforms of different converters are completely synchronized. Thus, there is no high frequency circulating current. In state 2 of FIG. 3, the high frequency voltage difference between the converters is the biggest when the switching waveforms of different converters are completely reversed. Thus, the high frequency circulating current is the biggest. The sliding frequency between these two states depends on the accuracy of the crystal-vibration, and it normally needs a few seconds to complete a sliding cycle. The disadvantage of this method is that the circulating current varies during a period of time, and in the aforesaid state 2 the circulating current is very big that would result in high ripple on the output voltage, and it varies. Thus, the filter capacitor current is big so as to cause overheating easily.
The third method is to use separate control for the parallel-connected converters, while the switching frequency is under variable frequency control. At present, this is the primary method used in the industry. That is to say the switching frequency of each converter keeps on changing such that during the period of the utility frequency most of the outputs of the converters are between the two states of FIG. 3. This, in fact, averages the high frequency circulating currents under the state 1 and state 2 of FIG. 3, and the amount of the circulating current depends on the filter inductance of the converters. The disadvantage of this method is the large variation of the switching frequency, and this may cause difficulty for the control of the converter. And the relatively big high frequency circulating current still exists.
The technical problem for the present invention to solve is to provide a method of switching synchronization of parallel-connected converter system. In the condition of digitized separate control and substantially constant switching frequency, the synchronized switching frequency is realized among the parallel-connected converters. The reduction of the high frequency circulating current among the converters may facilitate the realization of reliable parallel connection.
The technical embodiment of the present invention is to provide a method of switching synchronization of the parallel-connected converter system, characterized by setting one of the converters as the host converter, setting other remaining converters as slave converters. The host converter contains at least a first timer and a pulse-generating device, and the slave converter contains at least a second timer, a pulse-edge capture device, and a synchronization adjuster. The converters are connected through a synchronizing signal bus. The converter system is synchronized in accordance with the following steps.
The first timer of the host converter is used to send synchronization pulses toward the synchronizing bus through the pulse-generating device at a predetermined time Tk1.
The second timers of the slave converters record the time Tx of the edge of the synchronization pulses received by the pulse capture device from the synchronization bus.
The synchronization adjuster carries out the synchronization algorithm based on TK1 and Tx, adjusts the second timers of the slave converters to be synchronized with the first timer of the host converter. And then according to the predetermined protocol, the switching carrier timers of the slave converters are adjusted to synchronize with the switching carrier timer of the host converter.
In accordance with the method of the present invention, the first timer of the host converter also functions as the switching carrier timer when there is only one timer in each converter, and the second timer of each slave converter can function as the switching carrier timer as well. When the second timer of the slave converter is adjusted to be synchronized with the first timer of the host converter, the synchronization of the switching carrier timers of the host and slave converters is achieved.
In accordance with the method of the present invention, each converter may add an extra timer as its switching carrier timer, and both of the relationships between the first timer and the switching carrier timer of the host converter and between the second timer and the switching carrier timer of each slave converter follow a predetermined first protocol. The first protocol may set forth that the first timer of the host converter and its switching carrier timer maintain synchronized, and that the second timer and its switching carrier timer of the same slave converter also maintain synchronized. However, the first protocol may also set forth that the switching carrier timer of the host converter delays or advances a predetermined time from the first timer thereof, and the switching carrier timer of the slave converter delays or leads a predetermined time from the second timer thereof.
In accordance with the present invention, a second timer can be further added to the host converter, and a first timer can also be added to each of slave converters. Whilst the first and the second timers of the same converter maintain synchronization, and the relation of these two timers follows a second protocol. When the synchronization adjuster synchronizes the second timer of the slave converter with the first timer of the host converter, the first timer of the slave converter is also synchronized with the first timer of the host converter so as to realize the synchronization of the switching carrier timer of the host and slave converters.
In accordance with the present invention, when there are two timers in a converter, the second protocol will set the second timer at a continuous up or continuous down counting mode. If the first timer of the same converter is set as up/down counting mode, the period of the second timer thereof is twice as that of the first timer; and if the first timer of the same converter is set as continuous up or continuous down counting mode, the period of the second timer is the same as that of the first timer.
In accordance with the present invention, when there are two timers in a converter, the first timer of each converter can be used as the switching carrier timer of the same converter.
Application of the method of the present invention in a parallel UPS can effectively realize switching synchronization of the parallel-connected inverters, and suppressing the high frequency circulating current between the inverters.
The following is the detailed description of the embodiments of the present invention with the aid of the accompanying drawings.